Gem::Commands::SetupCommand#generate_default_dirs

# File tmp/rubies/ruby-3.3.0/lib/rubygems/commands/setup_command.rb, line 449
def generate_default_dirs
  prefix = options[:prefix]
  site_or_vendor = options[:site_or_vendor]

  if prefix.empty?
    lib_dir = RbConfig::CONFIG[site_or_vendor]
    bin_dir = RbConfig::CONFIG["bindir"]
  else
    lib_dir = File.join prefix, "lib"
    bin_dir = File.join prefix, "bin"
  end

  [prepend_destdir_if_present(lib_dir), prepend_destdir_if_present(bin_dir)]
end